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(B) The gravitational potential energy $U$ of a system is defined as the work done in assembling the system by bringing mass elements from infinity to

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$ - \frac{GM^2}{2R}$

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(B) The gravitational potential energy $U$ of a system is defined as the work done in assembling the system by bringing mass elements from infinity to their respective positions.For a thin spherical shell of mass $M$ and radius $R$,we consider building the shell by adding infinitesimal mass elements $dm$ to the surface.However,for a thin shell,the potential $V$ at the surface is constant and equal to $-\frac{GM}{R}$.When we assemble the shell,the total work done is $W = \int V dm = \int_{0}^{M} -\frac{GM}{R} dm$.Since the potential $V$ is constant for the entire shell of radius $R$,we integrate $dm$ from $0$ to $M$:$U = -\frac{G}{R} \int_{0}^{M} M' dm$ is not correct here; rather,we consider the work to bring the mass $M$ to the surface of a shell of radius $R$ where the potential is constant.The correct expression for the self-energy of a thin spherical shell is $U = -\frac{GM^2}{2R}$.

The self gravitational potential energy of a spherical shell of mass $M$ and radius $R$ is

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